Creatinine is an end product of a creatine pathway and has a structure in which creatine is dehydrated and cyclized. In the body, creatinine is present as creatine or creatine phosphate predominantly in the muscle. Creatine receives a high-energy phosphate from ATP to be transferred to creatine phosphate, which functions as an energy storage substance. Specifically, creatine phosphate provides a high-energy phosphate to ADP, and transfers to creatine or creatinine by a nonenzymatic reaction during energy consuming such as muscle contraction. Then, the generated creatinine is excreted into the urine through the kidney.
Therefore, an amount of creatinine excreted in the urine, i.e., a urinary concentration of creatinine is used as an indicator for muscle diseases or renal dysfunction. A blood creatinine level may also be an indicator for a disease such as renal dysfunction.
In the scene of group medical examination or in the situation of primary diagnosis such as screening examination, a dry test strip prepared by previously applying or impregnating a support with a reagent, which is colored by reacting with creatinine, and by drying the support has widely been used in order to quickly measure the urinary, serum, or plasma level of creatinine.
A dry test strip for measuring creatinine in accordance with the related art is prepared by utilizing a color reaction (Jaffe reaction) of a condensate of creatinine with picric acid under strong alkaline conditions. As one of such test strips, a test strip for measuring creatinine, which is composed of a support, a reagent layer containing 3,5-dinitrobenzoic acid carried on the support, and a development layer that is disposed on the reagent layer and contains a strong alkaline material such as lithium hydroxide is disclosed in Unexamined Japanese Patent Application KOKAI Publication No. H09-061430.
As the color reaction for measuring creatinine, an enzymatic method is also known. Japanese Patent No. 4243255 discloses a dry analysis element which quantifies gaseous ammonia generated by a reaction of creatinine with creatinine iminohydrase. Specifically, an indicator layer containing an indicator which produces a change detectable by gaseous ammonia, a liquid barrier layer through which the gaseous ammonia is passed, a reagent layer that contains an alkaline buffer and, as needed, can react with a matrix to generate ammonia, and a development layer are integrally layered on a transparent support in this order.
In addition, a creatinase-sarcosine oxidase-peroxidase method, which is one of enzymatic methods, has been utilized in wet creatinine measurement reagent kits in accordance with the related art. In “CRE-L Reagent Kainos Package Insert, Fourth Edition, Kainos Laboratories, Inc., 2010,” “N-Assay CRE-L Nittobo Package Insert, Eleventh Edition, Nitto Boseki Co., Ltd., 2008,” and “Exceliza CRE Package Insert, Sixth Edition, Sekisui Medical Co., Ltd., 2008,” wet creatinine measurement reagent kits including first reaction reagents and second reaction reagents are disclosed. The first reaction reagent predecomposes endogenous creatine in a sample by creatininase. The second reaction reagent is added into the sample after a reaction by adding the first reaction reagent, creatinine in the sample is converted into creatine by creatininase, the creatine is decomposed into sarcosine and urea by creatinase, the sarcosine is reacted with sarcosine oxidase to produce hydrogen peroxide, and a quinone pigment is produced by oxidative condensation of a hydrogen donor compound and 4-aminoantipyrine in the presence of the hydrogen peroxide and peroxidase.